In order to cope with recent regulations on the components of emission gas (exhaust gas) of an internal combustion engine, various types of exhaust gas purification units are disposed in the exhaust pipe passage of the internal combustion engine. One of these exhaust gas purification units is a selective catalytic reduction (SCR) unit, which has been known as an apparatus for reducing and purifying NOx. Such an SCR unit reduces and purifies NOx through use of an NOx reducing catalyst and ammonia produced as a result of hydrolysis of urea water serving as a reducing agent. A temperature of about 200° C. is needed for hydrolysis of urea water, and when exhaust gas is used as a heat source or when a heat source for hydrolyzing urea water is disposed within an exhaust passage, there is a problem in that when the temperature of exhaust gas is low, the urea water cannot be hydrolyzed. A technique for solving such a problem has been proposed (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2014-159776). According to the proposed technique, a heat source is disposed outside the exhaust passage so that even when the exhaust gas temperature is low, the heat for heating the reducing agent is not taken away. Also, there has been proposed a technique of providing a side stream of exhaust gas and supplying urea water to the side stream so as to hydrolyze the urea water through use of the entropy of gas (Japanese Patent Application Laid-Open (kokai) No. 2004-514828).
However, each of the conventional techniques is a technique of making up for heat loss produced by the flow of exhaust gas to thereby provide a temperature environment in which urea water can be hydrolyzed or a technique of efficiently utilizing the thermal energy of exhaust gas. In contrast, there is the fact that recent internal combustion engines have been improved in combustion efficiency, and therefore, the temperature of exhaust gas is unlikely to exceed a temperature at or above which urea water can be hydrolyzed.